1. Field of the Invention
The present invention relates to the maintenance of a vacuum within the space occupied by a high speed flywheel rotor. More specifically, the present invention relates to the use of a molecular pump incorporated into the flywheel assembly of a flywheel energy storage system to pump gases from a rotor environment into a separate chamber. The separate chamber advantageously can contain molecular sieves for adsorbing gas molecules given off by the rotor.
2. Brief Discussion of Related Art
Modern high strength-to-weight ratio fibers make it possible to construct high energy density flywheels, which, when combined with a high power motor-generators, are an attractive alternative to electrochemical batteries for use as energy buffers in hybrid electric vehicles. A properly designed flywheel system would provide higher energy density, higher power density, higher efficiency, and longer life than a conventional electrochemical battery. Flywheel energy storage systems have even been proposed for use in motor vehicles. U.S. Pat. No. 3,741,034, for example, discloses a flywheel contained in an evacuated sphere which is surrounded by a liquid.
The vehicle environment, however, presents special challenges to successful implementation of a flywheel to motor vehicle applications. For example, to accommodate a rim speed of about 1200 meters per second, a housing containing the flywheel should maintained at a very low pressure, e.g., a pressure in the below 0.001 Pascal, to limit windage losses. While this pressure can be readily achieved before sealing the housing, the fiber composite materials used in the construction of high energy density flywheels have a residual gas evolution rate which make it difficult to achieve this desired degree of pressure, i.e., near vacuum conditions, in a sealed container. Thus, continuous pumping of the evolving gases from the container is often needed. Most often, an external pump is employed to maintain the desired pressure.
U.S. Pat. Nos. 4,023,920, 4,732,529 and 4,826,393 describe various implementations of molecular pumps, which are a class of high vacuum pump wherein the dimensions of the critical elements are comparable to the mean free path of the gas molecules at the pressure of interest. Two types are generally known, a turbo-molecular pump, which is similar in construction to an axial flow compressor in a gas turbine employing interleaved rotor and stator blades, and a molecular drag pump, which uses helical grooves cut in the stator, which, in turn, is disposed in close proximity to a high speed rotor so as to direct gas flow through the pump. It will be appreciated that hybrid molecular pumps which pumps contains separate sections of each of these types or molecular pumps, are also known. More specifically, U.S. Pat. No. 4,023,920 discloses a turbo-molecular pump using magnetic bearings to support the pump rotor at high rotational speeds. U.S. Pat. Nos. 4,732,529 and 4,826,393 disclose hybrid molecular pumps in which a turbo-molecular section is used on the high vacuum input side and a spiral groove drag pump is used on the discharge side.
All of these pumps are designed as self contained systems, each with its own shaft, bearing system and power source, i.e., motor. While this solution is satisfactory for stationary systems, it is more difficult to apply in mobile applications because the space and weight for its implementation is not readily available.
The present invention was, thus, motivated by a desire to correct perceived problems in providing a flywheel-motor-generator energy storage system suitable for moving vehicles.